As I understand it, in the context of alexa, a 'skill' is an app.
SiriKit is used differently - from the docs:
SiriKit encompasses the Intents and Intents UI frameworks, which you use to implement app extensions that integrate your services with Siri and Maps. SiriKit supports two types of app extensions:
https://developer.apple.com/documentation/sirikit
>info about the free plan vs the paid plan
If you mean a paid Developer Program account vs. Xcode's free provisioning, see this thread [how i can have free Developer Program ] with an already moderated link that explains Xcode's free provisioning process (with screenshots) - be sure to read all the way thru it and any updates at the bottom.
Good luck.